In this type of exhaust system control apparatus, the sub-chamber is placed into operational mode by releasing the opening and closing valve, and the number of vibrations inherent in the exhaust line is thus reduced. As the exhaust line reaches an expanded state, the pulsation effect in the exhaust line is adjusted to fit the low-speed range operation of the engine, and the sub-chamber is put into non-operation mode by closing the opening and closing valve. The number of vibrations inherent in the exhaust line is increased just as the exhaust line reaches a contracted state and the pulsation effect in the exhaust line is adjusted to fit the high-speed range operation of the engine. Thus, by making use of the pulsation effect over the entire operational range, it is possible to heighten the intake efficiency of the engine and to enhance its performance.
The present inventors carefully analyzed the ratio between the cross-sectional areas of the exhaust line and the branch line in this type of exhaust system control apparatus, i.e., the considerable effect which the branching rate has on engine output performance. It was determined that, if the branching rate is too small, the sub-chamber does not operate effectively on the exhaust line even when the opening and closing valve is opened, due to the line resistance of the branching line, and accordingly, full enhancement of output performance is not obtained in the low-speed range of operation of the engine. By contrast, when the branching rate is too large, even when the opening and closing valve is closed, due to the excessive space in the branching line remaining in relation to the connection with the exhaust line, fluctuations in the inherent number of vibrations in the exhaust line occur, and output performance is lowered in the high-speed range of operation of the engine.